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Palmer Amaranth, a New Threat to Pennsylvania Agriculture

Palmer amaranth is a species of pigweed that was recently introduced into Pennsylvania. It was identified on at least 7 farms in 2013 and poses unique management challenges.

Containing new infestations and preventing its spread is a critical first step to managing this new threat. The risk from this new weed comes from its competitive growth habit, prolific seed production (greater than 100,000 seeds per plant) along with its potential resistance to glyphosate (e.g. Roundup) and the Group 2 herbicides (ALS-inhibitors). Resistance to other herbicide classes has also been documented in this species (Groups 3 and 5).

Palmer amaranth (Amaranthus palmeri) is a summer annual broadleaf weed that is native to the southwestern US and Mexico. It is also known as Palmer pigweed. Palmer amaranth is related to other pigweeds in our region including redroot, smooth, Powell, and spiny, but unlike these other pigweeds, Palmer amaranth grows faster and is dioecious, meaning that plants are either male or female. Pollen from male plants can travel with the wind to susceptible female plants and if the male is herbicide resistant, a portion of the offspring will also be resistant. Waterhemp (Amaranthus tuberculatus), another dioecious herbicide resistant pigweed species common in the Midwest is also getting a foothold in Pennsylvania. Although not as great a competitive threat as Palmer amaranth, it too should be aggressively managed to prevent its spread.

Use an integrated management approach that includes soil residual herbicides and effective postemergence control. Small plants that are less than 4 inches tall are easier to manage.

With smaller infestations, physically remove plants from the field prior to flowering.

For flowering plants, determine if the plants have set viable seed. This will be important for determining the future of the problem. If no viable seed is yet present (darkened seeds that shatter), hand rogue smaller infestations and remove plants from within field. Bury or burn removed plants along field edge. Manage the field with no-till if possible leaving any potential seeds near the soil surface. This should enhance seed predation and mortality.

If the problem is too large to handle by hand or if viable seed are present, please consult with Penn State Extension and/or a Professional Crop Advisor to determine the best management strategy.